Defense trade-offs in ripe fruits: Secondary compounds deter pests but reduce consumption by mutualist seed dispersers

Background/Question/Methods

Fleshy fruits attract animal seed dispersers by providing nutritional rewards; however, these rewards can also attract fruit pests such as insect seed predators and fungal pathogens.  Thus, fruits must be both attractive to mutualists and defended against antagonists at the same time.  Fruit secondary chemistry is likely of central importance in mediating the triad of interactions among plants, seed dispersers, and fruit pests, and consequently the reproductive success of many plant species.  However, few studies have provided any information on the quantitative variation in fruit secondary chemistry or its ecological consequences.  To better understand how fruit secondary chemistry can influence seed dispersal and fruit defense, I conducted a series of field and lab experiments focused on the role of amides, a large class of N-containing defensive compounds, in the interactions among Neotropical pepper plants (Piper spp., Piperaceae), their primary seed dispersers (bats in the genus Carollia), and several associated pests, including insects and fungal pathogens. 

Results/Conclusions

My results show that amides reduce the growth rates of fruit-associated fungi in a dose-dependent manner and have deterrent effects against a specialist insect seed predator, Sibaria englemani (Hemiptera, Pentatomidae).  In addition, behavioral experiments with three species of Carollia bats indicate that amides reduce fruit consumption by two species (C. perspicillata and C. sowelli), but do not affect the preferences of a third (C. castanea).  Of the three bat species, C. castanea is the most specialized on Piper, which may explain an increased tolerance for amides in this species.  Together, these results suggest that amides in Piper fruits likely represent an adaptive trade-off between attraction of seed dispersers and defense against fruit pests. Additional work will focus on the variation in fruit amide content among Piper species, since large differences in the dispersal ecology of different Piper species (e.g. variable levels of dependence on bats as seed dispersers) may lead to different selective regimes that would shift the balance of costs and benefits of fruit secondary compounds in the different species. By applying the tools of organic chemistry to better understand the complexities of fruit/frugivore interactions in this model genus, this work can provide valuable insights for both plant defense theory and our understanding of seed dispersal mutualisms.